This invention relates to a process for making steel having enhanced precipitation strengthening and to high-strength micro-alloy steel made by means of the process.
Many of the industrially-significant attributes of different steels (strength, hardness etc.) depend in part on the microstructure of the particular steel, that is the type or types of crystals of which the steel is composed and the grain size of the crystals. In typical steel manufacturing, the steel undergoes processing in order to produce a desired microstructure. Such processing typically includes thermal processing (including controlling the cooling rate of the steel to promote the formation of particular crystal structures in the steel) and mechanical processing (including reducing the thickness of the steel by rolling the steel, so as to, for example, cause recrystallization for the purpose of reducing the grain size of the steel). The attributes of a steel can also be affected by the addition of precipitation strengthening substances, that is, alloying substances that dissolve when the steel is heated and then tend to precipitate in the boundaries between the grains of the steel when the steel cools. The precipitate particles thus created build up resistance to slip between steel grains, thereby increasing the strength of the steel, particularly the yield strength.
The known precipitation strengthening substances suitable for use in steel include niobium (referred to at times herein as Nb), titanium (referred to at times herein as Ti) and vanadium (referred to at times herein as V). Niobium typically combines with carbon (referred to at times herein as C) and possibly nitrogen (referred to at times herein as N), and precipitates as Nb(C,N) and/or NbC. Titanium typically combines with carbon and precipitates as TiC. Vanadium typically combines with nitrogen or carbon and precipitates as VN or VC. Niobium, titanium and vanadium may be present in steel for purposes other than direct precipitation strengthening and will, during typical steel production, combine with other alloying substances in the steel, but the above compounds (Nb(C,N), NbC, TiC, VN and VC) are those that are considered to be associated with, and significant for, ultimate precipitation strengthening. Titanium will also form TiN with nitrogen, but this is not a useful precipitation strengthening compound, largely because TiN forms and precipitates at relatively high temperatures, resulting in larger-than-desired precipitate particles (discussed generally in what follows). Various other possible precipitation strengthening compounds are also known, including: Ti(C,N), V(C,N) and TiNb(C,N).
The extent to which the addition of such precipitating substances increases the strength of the steel depends in part on the ultimate size and volume fraction of the precipitate particles. It is well known that the strengthening effect of such precipitation increases as the volume fraction of the precipitate particles increases and the precipitate particle size decreases. For a given volume fraction of precipitates, a smaller particle size means a higher number density of precipitate particles, that is, a higher number of interactions between precipitate particles and steel grains, and thus higher strength. With Nb(C,N) and/or NbC precipitation strengthening in ferrite steel, for a given volume fraction, the increase in yield strength attributable to precipitation strengthening increases by about one order of magnitude when the precipitate particle size is reduced from about 100 nm to about 3 nm.
For a given precipitating substance, precipitate particle size is primarily dependent on the temperature at which the particles form. Generally, the lower the temperature at which the precipitate particles form, the smaller the particle size. The volume fraction of the precipitate particles depends in part on the rate at which the precipitating substance diffuses within the solid metal. Generally the rate of diffusion is a function of temperature; a higher temperature resulting in a higher diffusion rate and thus a higher volume fraction of precipitate particles.
For some metals and some precipitating substances, the diffusion rate of the precipitating substance is sufficiently high at relatively low temperatures, (for example, room temperature) that significant precipitation strengthening occurs at these relatively low temperatures. Precipitation strengthening that occurs over time at room temperature, referred to as aging, generally produces relatively fine precipitate particles. For steel, the diffusion rate of the known precipitating substances is too low at room temperature to produce an appreciable volume fraction of precipitate particles, which means that aging does not result in significant precipitation strengthening. For example, although Nb(C,N) and/or NbC precipitation is thermodynamically possible in ferrite steel at relatively low temperatures, such as below about 500 C., because of the sluggish precipitation kinetics at these temperatures, only a minimal Nb(C,N) and/or NbC precipitation strengthening effect has been observed at these temperatures under industrial conditions.
It is known to reheat metals containing precipitating substances off-line to increase the rate of diffusion of the precipitating substances and thus increase the volume fraction of the precipitate. However, off-line heat treatment is generally not an effective way to enhance precipitation strengthening in steel. For steel and the precipitating substances known to be appropriate for steel, re-heating the steel to a temperature sufficiently high to increase the diffusion rate of the precipitating substance so as to increase the volume fraction of the precipitate particles within a commercially-reasonable period of heating time, generally results in a larger-than-desirable precipitate particle size. As well, off-line heat treatment of steel is costly and typically, and significantly, results in a loss of desirable microstructure characteristics of the steel. Therefore, off-line heat treatment is typically not the best technique for enhancing precipitation strengthening in steel.
What is needed is a process that increases the volume fraction of fine precipitates in steel so as to result in enhanced precipitation strengthening.
In accordance with an aspect of the present invention, there is provided a process for producing steel having a desired microstructure, and precipitation strengthening particles of a desired particle size and volume fraction for enhanced precipitation strengthening, the process including the steps of:
a) heating steel containing a precipitation strengthening substance to a selected dissolving temperature selected to dissolve substantially all of the precipitation strengthening substance in the steel;
b) processing the steel to produce the desired microstructure;
c) cooling the steel to a selected target temperature at which the desired microstructure is essentially stable and at which those precipitation strengthening particles that form tend to be of the desired particle size; and
d) with the steel at the selected target temperature, deforming the steel to introduce dislocations into crystal structure of the steel so as to increase the kinetics of precipitation, and thus the volume fraction, of precipitation strengthening particles of the desired particle size.
Note that if the steel is being made as part of an on-line processing operation involving rolling after, say, continuous casting optionally followed by reheating, steps (a) and (b) can be conventional in character, and no special subsequent heating and processing steps are required before steps (c) and (d) are taken; the as-rolled steel can be cooled to a selected temperature pursuant to step (c) and then deformed pursuant to step (d).
Introducing dislocations in the crystal structure of steel is understood to increase the kinetics of precipitation by both: increasing the number of nucleation sites; and to increase the kinetics of diffusion in that vacancies in the crystal structure associated with the dislocation of the crystal structure accelerate the diffusion of the precipitating substances.
The precipitating substance may be any suitable precipitation strengthening substance, including niobium, titanium or vanadium, or combinations of suitable substances. A skilled metallurgist will be able to determine appropriate precipitating substances having due regard to the desired characteristics of the end product.
Advantageously, for many precipitation strengthening substances, the selected target temperature (that is the temperature at which the precipitate particles that form tend to be of a selected target size desirable for precipitation strengthening) is a temperature at which the microstructure of steel is essentially stable. Thus, enhanced precipitation strengthening can be achieved through deforming the steel at the target temperature without loss of desirable microstructure features of the steel.
The steel may be deformed by bending or rolling the steel or by any other means appropriate for steel when at the selected target temperature.
Preferably, the time period between the time when the steel is heated to dissolve the precipitation strengthening substance, and the time when the steel is at the target temperature and the dislocations are introduced, is kept as short as possible (subject to the time required for any thermomechanical processing required to produce a desired microstructure) so as to minimize the formation of precipitate particles of larger than the desired target size. As is well known, the precipitate particles that form at higher temperatures tend to be larger than those that form at lower temperatures. Such larger-than-desired precipitate particles are not as effective at precipitation strengthening as precipitate particles of the smaller, desired target size, and the formation of such larger-than-desired precipitate particles consumes precipitation strengthening substance that would otherwise be available for precipitation at the desired target temperature.
In accordance with another aspect of the present invention, there is provided a more detailed process conforming generally with the previously defined process, for making a steel having enhanced precipitation strengthening. The process is preferentially applicable to the production of high-strength micro-alloyed structural steels, and pressure-vessel or line-pipe-grade steels. In a preferred embodiment of the process, the steel to which the process is applied is low-carbon for good weldability. The steel may also contain other alloying elements such as manganese and molybdenum for purposes other than precipitation strengthening. The steel-making process includes the steps of:
a) heating steel containing a precipitation strengthening substance to a selected dissolving temperature selected to dissolve substantially all of the precipitation strengthening substance in the steel;
b) with the steel at a temperature above the temperature below which austenite does not recrystallize (Tnr), breaking down the austenite grains through multiple recrystallization cycling to produce an austenite grain size of about 30 m or less;
c) with the steel at a temperature below the Tnr but above the temperature at which austenite begins to change to ferrite on cooling (Ar3), producing a heavily pancaked austenite structure in the steel;
d) cooling the steel at a rate of about 15 C./sec to about 20 C./sec from a temperature above the Ar3 to a stop-cooling temperature between about 350 C. and about 450 C.; and
e) with the steel at a temperature between about 350 C. and about 450 C., deforming the steel to introduce dislocations in the crystal structure of the steel so as to enhance precipitation of the precipitating substance.
The heating of the steel in step a) above, is to a temperature sufficiently high to dissolve substantially all of the precipitating substances. Preferably, the steel is heated about 50 C. higher than the estimated equilibrium solution temperature of the precipitating substances to ensure that substantially all of the precipitating substances are dissolved. However, the steel may be heated to a temperature closer to the equilibrium solution temperature although it may take longer to dissolve substantially all of the precipitating substances at temperatures below 50 C. above the equilibrium solution temperature. At temperatures above 50 C. above the equilibrium solution temperature, dissolving substantially all of the precipitating substances would take less time. Too high a temperature will result in the grain size being undesirably coarsened. Depending on the steel chemistry, a temperature of at least about 1050 C. and no more than about 1350 C. may be appropriate. The steel that is heated may be in the form of a previously-cast slab, such that the heating of the steel involves reheating the slab, in conformity with conventional steel mill practice. However, it may be that the slab is received from the caster at the desired sufficiently high temperature, such that the heating of the steel in step a) results, in the as-cast steel, from the casting of the steel, and in such case it is not necessary as a separate discrete heating step to reheat the slab to the desired temperature.
An appropriate temperature at which to break down the austenite grains through multiple recrystallization cycling, as referred to in step b) above, may at least be slightly higher than the Tnr and no more than about 1200 C. The breaking down of the austenite grains may be through multiple recrystallization cycling by rolling the steel for a series of reducing roughing passes, such as in a Steckel mill having associated coiler furnaces. Preferably the temperature of the steel for the first roughing pass is about 1200 C. and the temperature of the steel for the last roughing pass is slightly higher than the Tnr. The use of a Steckel mill with associated coiler furnaces to facilitate multiple recrystallization cycling has been previously described, for example in Dorricott U.S. Pat. No. 5,810,951, granted on Sep. 22, 1998. The roughing passes cause recrystallization of the steel by deforming the steel so as to introduce dislocations that are stored in the structure of the steel, making the microstructure unstable and creating grain nucleation sites in the boundaries between the grains. As is well known to persons skilled in the art of metallurgy, since the steel is above the Tnr, by definition the temperature above which austenite will recrystallize, new strain-free grains will tend to form in the grain nucleation sites. If the number density of the stored dislocations is high enough, the new grains will grow and gradually replace the deformed grains. The newly formed grains will tend to have a higher number density and smaller grain size than grains formed earlier in the process. When a new deformation-recrystallization cycle starts, these grains will provide more nucleation sites for the xe2x80x9cnext generationxe2x80x9d grains. Each roughing pass will introduce new grain nucleation sites and thus promotes the formation of additional grains. In this way, multiple roughing passes, and a multiple cycle of deformations, increases the number of nucleation sites and thus grains, and reduces the average size of the grains.
The steel temperature below the Tnr but above the Ar3 (referred to above in step (c)) may be achieved by merely exposing the steel to air of ambient temperature, such as by removing the steel from the Steckel mill and associated coiler furnaces, if such are used in the rolling of the steel. Depending on the steel chemistry, the Ar3 temperature may be roughly 780 C. The heavily pancaked austenite structure, as referred to in step c) above, may be produced by rolling the steel (such as in a Steckel mill with associated coiler furnaces) in the temperature range of between the Tnr and the Ar3 for sufficient finishing passes to reduce the steel thickness by preferably about 70%. The steel temperature for the finishing passes should be at least about 20 C. higher than the Ar3 and no higher than about 50 C. less than the Tnr. Preferably, the steel temperature for the first finishing pass is about 50 C. less than the Tnr and the steel temperature for the last finishing pass is about 20 C. higher than the Ar3.
Any precipitation of the precipitating substances that occurs while the steel is in the austenitic region (that is, at temperatures above the Ar3) contributes little to the ultimate strength of the steel, in that the resulting precipitate particle size is larger than desired for optimum precipitation strengthening. It is preferable to minimize the coarser precipitates formed at higher temperatures so as to preserve the precipitation material for low temperature precipitation. Thus it is preferable to keep the steel at temperatures above the Ar3 for as short a time as possible. The speed at which the roughing passes step can occur is typically not limited by current mill technology, but is limited by the necessity of providing sufficient time between roughing passes for a desired amount of recrystallization to occur. The time between roughing passes depends in part on the steel chemistry, the grain size and the reduction for each roughing pass. A person skilled in the art of metallurgy will be able to determine an appropriate time between roughing passes. It is desirable to complete the finishing passes as rapidly as mill conditions permit.
Preferably the deforming of the steel (referred to above in step (e)) is by introducing bending strains into the steel or by a rolling reduction of the thickness of the steel. A relatively small deformation, for example a sustained strain of about 0.1 has been observed to accelerate the precipitation process by about two orders of magnitude. The increase in precipitation kinetics resulting from a relatively-low-temperature plastic deformation of the steel produces an appreciable volume fraction of extremely fine precipitate particles and consequently, significant precipitation strengthening. In the temperature range referred to in step (e) above (i.e. at least about 350 C. and no more than about 450 C.), the rate of precipitation of the known precipitating substances would normally be relatively low. However, it is understood that introducing dislocations in the crystal structure of the steel facilitates precipitation strengthening by both increasing the number of nucleation sites and accelerating the diffusion rate of the precipitating substances. In this temperature range the microstructure features of the steel are essentially stable, such that enhancing precipitation strengthening by deforming the steel while it is in this temperature range will not unacceptably detrimentally affect the microstructure of the steel.
If the steel is in plate form, the roughing passes, finishing passes and accelerated cooling will tend to introduce imperfections into the steel in the form of bends or ripples. Preferably such plate is deformed by the introduction of bending strains in the plate such as by being passed through a hot leveller to level (or straighten) the plate. The number of dislocations thus introduced in the steel depends on the total bending strain introduced by the hot leveller. It has been observed by the inventors that, if a plate being levelled is subjected to a total strain of about 4 to about 5 yield strains, the number density of dislocations is sufficient to produce significant precipitation strengthening. The inventors expect that a total strain in the range of about 1 to about 7 yield strains would be suitable for enhancing precipitation strengthening. For bending deformation such as introduced by a hot leveller, the maximum suitable deformation is clearly less than the deformation that would cause cracks to form in the steel.
Alternatively or additionally to being levelled, the steel may be deformed by being passed through a final-pass roller for a final rolling reduction pass. The inventors expect that if the steel is not also levelled, a final rolling reduction in the range of about 1% to about 5% would be effective to enhance precipitation strengthening. As well as enhancing precipitation strengthening, a final rolling reduction of at least about 1% and no more than about 5% would improve control of the final gauge of the steel and improve the surface quality of the steel.
It will be apparent to skilled metallurgists that various other methods for deforming the steel so as to introduce dislocations in the crystal structure of the steel could be used to obtain enhanced precipitation strengthening.
For steel having the following chemistry:
at least about 0.01 and no more than about 0.1% wt. carbon;
at least about 0.03 and no more than about 0.12% wt. niobium;
at least about 0.008 and no more than about 0.03% wt titanium;
at least about 1 and no more than about 1.9% wt. manganese;
at least about 0.1 and no more than about 0.5% wt. molybdenum;
a maximum phosphorus content of about 0.02% wt.;
a maximum sulfur content of about 0.015% wt.;
a maximum nitrogen content of about 0.015% wt.; and
the balance being iron (Fe) and incidental impurities;
The above-described steel-making process produces steel with a microstructure of about 30% polygonal ferrite and about 70% acicular ferrite with an average grain size of about 5 m or less; and having precipitate particles of NbC and Nb(C,N) with a precipitate particle size of generally less than about 5 nm and probably in the range of about 1 to about 3 nm.
Carbon is kept low in this steel for good weldability. As well, with respect to precipitation strengthening by the formation of NbC, only a very small amount of carbon is required for the purpose of combining with niobium because of the stoichiometric ratio (Nb/C=7.74). Thus, the inventors predict that the amount of carbon required to be present in the steel may be less than the rough minimum set out above.
Titanium is present in this steel to increase castability and to prevent grain growth during high-temperature reheating. Titanium is known to be an effective micro-alloying element for retarding grain coarsening. Titanium combines with nitrogen to form TiN which is stable at temperatures as high as about 1300 C. and can effectively retard the migration of grain boundaries. Thus TiN is effective for grain growth prevention over a large temperature range. Other alloying elements could be present in the steel to prevent grain growth but the known alternatives are not viewed as being as effective as titanium and/or are more expensive than titanium. For example, niobium can be used to form Nb(C,N) precipitation to prevent grain growth during high temperature reheating. However, at a temperature higher than about 1200 C., unless an unusually large, and therefore probably prohibitively-expensive, amount of niobium were present in the steel, most of the Nb (C,N) would dissolve into the steel matrix and would be ineffective in terms of retarding grain growth.
If too little titanium is present in this steel, the titanium may not be effective to prevent grain growth. If too much titanium is present, it may result in reduced toughness of this steel, particularly if the amount of nitrogen in the steel is relatively high. Preferably at least about 0.008 and no more than about 0.03% wt of titanium is present in this steel. More preferably, at least about 0.015 and no more than about 0.02% wt of titanium is present in this steel. Even more preferably, about 0.018% wt of titanium is present in this steel.
Manganese and molybdenum are present in this steel primarily to facilitate the formation of the desired microstructure. In particular, molybdenum acts with niobium to synergistically suppress the formation of polygonal ferrite and promote the formation of acicular ferrite. As well, manganese and molybdenum tend to impede the precipitation of Nb(C,N) in austenite and thus increase the amount of niobium available to precipitate at lower temperatures in ferrite, by both increasing the solubility of Nb(C,N) in austenite, and decreasing the rate of diffusion of niobium in austenite. Preferably at least about 1.4 and no more than about 1.9% wt of manganese is present in this steel. Preferably, at least about 0.1 and no more than about 0.5% wt of molybdenum is present in this steel.
The concentrations of phosphorus, sulfur and nitrogen are compatible with melting the steel in electric arc furnaces. The maximum phosphorus content of the steel is about 0.02% wt. More preferably, the maximum phosphorus content of the steel is about 0.018% wt. The maximum sulfur content of the steel is about 0.015% wt. More preferably, the maximum sulfur content of the steel is about 0.01% wt. The maximum nitrogen content of the steel is about 0.015% wt. More preferably, the maximum nitrogen content of the steel is about 0.013% wt.
The incidental impurities present in the steel may include miscellaneous non-essential elements, having, when present in sufficient quantity, an alloying effect on steels containing them, but whose effect on the steels described herein is innocuous.
As set out above, it is well known that various alternative precipitate-forming substances undergo precipitation in a manner similar to NbC and Nb(C,N) and thus, as with NbC and Nb(C,N), the kinetics of precipitation of these alternative precipitate-forming substances is expected to be increased by the introduction of dislocations into steel containing these alternative precipitate-forming materials. It will accordingly be clear to skilled metallurgists that precipitate-forming substances other than niobium may be present in this steel, including but not limited to: vanadium (to combine with nitrogen or carbon to form VN or VC); and titanium (to combine with carbon to form TiC).
The tendency of titanium to combine with nitrogen at relatively high temperatures means that titanium is not effective for enhanced precipitation strengthening unless the amount of nitrogen in the steel is relatively low, that is, the steel has a maximum nitrogen content of about 0.005% wt. Otherwise, much of the titanium will be consumed at higher temperatures, that is, it will combine with nitrogen and as a result not be available to perform a precipitation-strengthening function in the steel. A suitable chemistry for a steel having titanium as the significant precipitating substance for precipitation strengthening (and therefore being relatively low in nitrogen) is as follows:
at least about 0.01 and no more than about 0.1% wt. carbon;
at least about 0.03 and no more than about 0.15% wt. titanium;
at least about 1.0 and no more than about 1.9% wt. manganese;
at least about 0.1 and no more than about 0.5% wt. molybdenum;
a maximum phosphorus content of about 0.02% wt.;
a maximum sulfur content of about 0.015% wt.;
a maximum nitrogen content of about 0.005% wt.; and
the balance being iron (Fe) and incidental impurities.
A suitable chemistry for a steel having niobium and/or titanium as the significant precipitating substance for precipitation strengthening (and therefore also being relatively low in nitrogen) is as follows:
at least about 0.01 and no more than about 0.1% wt. carbon;
at least about 0.03 and no more than about 0.15% wt. titanium and a maximum niobium content of 0.12% wt., such that the total combined amount of titanium and niobium is at least about 0.03 and no more than about 0.2% wt.;
at least about 1.0 and no more than about 1.9% wt. manganese;
at least about 0.1 and no more than about 0.5% wt. molybdenum;
a maximum phosphorus content of about 0.02% wt.;
a maximum sulfur content of about 0.015% wt.;
a maximum nitrogen content of about 0.005% wt.; and
the balance being iron (Fe) and incidental impurities.
Vanadium may be present in the steel as a precipitating substance either in addition to niobium, or as an alternative to niobium. If niobium and vanadium are both present in the steel for precipitation strengthening, the total amount of these two substances should not exceed about 0.2% wt. Since one of the desired precipitating compounds of vanadium contains nitrogen (VN) and titanium tends to combine with nitrogen at relatively high temperatures (thus potentially using up much of the titanium and the nitrogen), if vanadium is being present in steel for precipitation strengthening, the amount of titanium in the steel should be no greater than about 0.03% wt. A suitable chemistry for a steel having niobium and/or vanadium as the significant precipitating substance for precipitation strengthening is as follows:
at least about 0.01 and no more than about 0.1% wt. carbon;
a maximum niobium content of about 0.12% wt. and a maximum vanadium content of about 0.12% wt., such that the total combined amount of niobium and vanadium is at least about 0.03% wt. and no more than about 0.2% wt.;
at least about 0.008 and no more than about 0.03% wt. titanium;
at least about 1.0 and no more than about 1.9% wt. manganese;
at least about 0.1 and no more than about 0.5% wt. molybdenum;
a maximum phosphorus content of about 0.02% wt.;
a maximum sulfur content of about 0.015% wt.;
a maximum nitrogen content of about 0.015% wt.; and
the balance being iron (Fe) and incidental impurities.
The various features of novelty that characterize the invention are pointed out with more particularity in the claims. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be made to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention.